1. Technical Field
The present invention relates generally to lubricants and more particularly to solid lubricants containing bismuth sulfide for use in friction linings for applications such as, but not limited to, brake disk pads, brake drums, and clutch disks.
2. Discussion
Friction linings such as those typically employed in brake linings and clutch linings are usually comprised of either asbestos fibers, mixtures of asbestos fibers and other heat resistant inorganic or organic fibers, asbestos-free mixtures of heat resistant inorganic or organic fibers, or metal powders such as iron powder, copper powder, steel powder or mixtures thereof, in combination with an organic monomeric or polymeric binder system (e.g., phenolic or cresylic resin). Because asbestos has been alleged to be the cause of certain health problems and is no longer environmentally acceptable, most modern friction linings are made without asbestos. Thus, most current friction linings are made from synthetic and steel fibers, and iron, ceramic, and metallic powders.
A typical friction lining formulation may optionally contain one or more of the following components: thermosetting resinous binders (e.g., phenolic resins such as phenol-formaldehyde resins, epoxies, and the like) present in conventional amounts; reinforcing fibers (e.g., aramid, steel, acrylic, and the like) present in conventional amounts; metal powders (e.g., iron, copper, brass, zinc, aluminum, antimony, and the like) present in conventional amounts; solid lubricants (e.g., molybdenum disulfide, graphite, coke, stannic sulfide, antimony trisulfide, and the like) present in conventional amounts; abrasives (e.g., tin oxide, magnesia, silica, iron oxide, alumina, rutile, and the like) present in conventional amounts; organic fillers (e.g., rubber particles, cashew nut shell particles, nitrile rubber particles, and the like) present in conventional amounts; and inorganic fillers (e.g., barytes, gypsum, mica, and the like) present in conventional amounts. Other materials may be added as well, as is known in the art.
As noted, both asbestos-containing as well as asbestos free friction linings may contain one or more solid lubricants in order to improve the tribological properties of such materials, particularly their friction and wear properties.
The known solid lubricant combinations and the friction materials prepared with them, fiber based as well as metal based, are on the whole not entirely satisfactory with respect to their tribological properties. For example, friction materials having satisfactory wear properties and adequate narrow coefficients of friction can be prepared with commercially available solid lubricants. However, these prepared friction materials do not offer vibration-free operation nor do they adequately prevent the transfer of friction material to the metallic friction surfaces. In addition, these solid lubricant combinations cannot always be incorporated with equally good results in the various compositions for preparing friction materials. They therefore cannot be universally incorporated into friction materials based on asbestos fibers, other fibers or metal powders.
Another concern with the known solid lubricant combinations and the friction materials prepared with them, is the level of noise or "equal" produced when automobile operators actuate the braking systems. For example, automotive brake part (e.g., brake pads) manufacturers are particularly interested in noise-reducing lubricants. One such lubricant which reportedly has noise-reducing qualities is lead sulfide. However, lead sulfide is not environmentally acceptable, and therefore is not well-suited for use as a friction material lubricant.
Therefore, there exists a need for a solid lubricant combination which is environmentally acceptable, reduces the noise levels of friction materials, can be used as an additive to friction materials in order to improve their tribological properties, which can be universally incorporated in a variety of compositions for the preparation of friction materials, especially in asbestos-free, semi-metallic, and/or low-metallic friction materials, and which yields an improved friction material having low wear properties and narrow coefficients of friction which, above all, offer vibration-free operation, and which do not result in the transfer of friction materials to the opposing friction plane of the article in question such as, for example, a brake disk, brake drum, or a clutch plate.